"Eiken" is a comedy anime series that revolves around the daily lives of the members of the Eiken Club, an extracurricular group at Shoubun Academy. The club is known for its eccentric and larger-than-life characters, particularly its female members, who possess exaggerated physical traits. The protagonist, Densuke Mifune, is an ordinary high school student who becomes entangled in the club's antics after being recruited by Chiharu Shiiya, the club's president. Chiharu is a petite but strong-willed girl who often drags Densuke into bizarre and chaotic situations.

The series primarily focuses on the interactions between Densuke and the club members, including the shy and reserved Kirika Nogame, the energetic and tomboyish Haruko Kobayakawa, and the enigmatic and mature Ruriko Tachibana. Each episode features comedic scenarios that highlight the absurdity of the club's activities, often involving slapstick humor, exaggerated reactions, and over-the-top situations. The narrative does not follow a strict overarching plot but instead presents a series of episodic adventures that emphasize the quirky dynamics between the characters.

Throughout the series, Densuke struggles to maintain a sense of normalcy while navigating the unpredictable and often surreal world of the Eiken Club. The humor is derived from the stark contrast between Densuke's grounded personality and the outrageous behavior of the club members. Despite the lack of a central storyline, the series maintains a lighthearted and comedic tone, with occasional moments of character development that provide insight into the personalities and motivations of the main cast. The anime's focus on humor and character interactions makes it a straightforward and episodic viewing experience, centered on the absurd and exaggerated nature of the Eiken Club's daily life.